A brief technical discussion is believed desirable to place the significance of this invention into proper perspective.
Current ship hulls are made of steel which is magnetic. Additionally, the present shipyard design uses the conventional single-hull construction with longitudinal stringers and transverse framing. To achieve a non-magnetic capability, stainless steel hulls are recently being investigated for the next generation of Navy ships. Furthermore, to achieve lower costs in connection with the use of stainless steel, a new advanced double-hull concept is being addressed. The double-hull concept also results in increased ship survivability. However, residual welding stresses lead to large plate (dishing) deformations during the fabrication process of steel hulls. These deformations which are called “hungry horse,” increase the hull's detection. Stainless steel hulls are expected to result in much higher residual stresses and, hence, in much higher “hungry horse” deformations. The only means to assure tight manufacturing tolerances is to relieve the residual stresses by heat treatment which is very expensive, or to use some advanced welding technology such as laser welding, that could minimize the residual stresses. However, such advanced welding technologies are normally not available at shipyards. The best alternative is to build the hull out of composites which permits the achievement of very tight dimensional tolerances. However, several studies have shown that for hulls longer than 200 feet, even carbon fiber composites would not provide the required stiffness and compressive strength that are required for the hull. Additionally, the cost of carbon fiber composites is prohibitive for this size of ships with the current cost of $12 to $18/lb. for carbon fiber compared to $0.45/lb. for high strength steel and $3/lb. for stainless steel. Known low-cost/high-performance composite materials, such as glass fiber composites (GRP) using resin transfer molding processing, that are now being used in patrol boats, corvettes and mine hunters, do not have the stiffness nor the in-plane strength required for long hulls of combatant ships or other large commercial ships. The load-carrying mechanism for long Navy combatants is by axial tension and compression in the hogging and sagging mode between waves. The in-plane strength of the composites therefore becomes the critical design factor. For small ships or boats, the bending strength of the composites is critical. The technology of known composite sandwich construction, common in connection with smaller ship lengths or boats, would not add to the carrying capability for sea loads in long ship hulls. GRP composites, however, are the best choice to achieve all of the magnetic, radar cross section and hydrodynamic signature requirements as well as low maintenance costs.
Composite hulls for Naval vessels of lengths less than 300 feet are presently being built using GRP or carbon fiber sandwich constructions that may use a patented process called “SCRIMP,” U.S. Pat. No. 4,902,215 and U.S. Pat. No. 5,958,325 or other room-temperature curing processes. In such prior art constructions, the entire hull is made of the same material which is very different from a hybrid construction where more than one material is used. In addition, this type of construction would not be able to sustain the sea loads for curved mid-body hulls for large ships of a length greater than 300 feet.
A composite-type hull construction that combines composites and steel is disclosed in the U.S. Pat. No. 4,365,580 to Blount and by others remotely related to Blount's patent. These other patents which are referenced in Blount are sandwich-type constructions wherein a synthetic foam material is sandwiched between inner and outer shells and hence are not hybrids of two different materials.
In the U.S. Blount Pat. No. 4,365,580, a steel hull construction is used consisting of an inner box-like structure with a fiberglass outer hull. The steel box is carrying all the sea loads (bending moments and shear), while the composite shell and foam transmits the water pressure to the box. Thus, the hull of this patent resembles a steel hull covered with an add-on parasitic composite skin that gives it the shape. This patent as well as the patents cited therein thus represent sandwich-type constructions in which a synthetic foam material is sandwiched between inner and outer shells and therefore are not hybrids of two different materials.
The U.S. Pat. No. 5,778,813 to Kennedy addresses a composite laminated panel for containment vessels such as double-hull oil tankers. It is composite in the sense that it is a steel double-hull with an elastomer core inbetween. However, this patent is not concerned with the problems addressed by the present invention because the steel carries all sea loads and the elastomer merely acts in shielding the inner hull from cracks when the outer hull is pierced, ruptured or penetrated. The U.S. Pat. No. 6,505,571 to Critchfield et al. describes some types of connections between composite and steel hybrid constructions which can be used in conjunction with hull constructions as disclosed in my prior patent. The main focus of the Critchfield patent is the connection between two different sections; namely, a fiber-plastic and a metallic hull section, whereas the instant invention relates to hulls with a curved mid-body section made of composites with light framing on the inside thereof for the mid-body section that transmit the sea loads to the longitudinal framing or the bulkheads.
My prior U.S. Pat. No. 6,386,131 incorporates the aforementioned key performance characteristics and requirements. However, the hull of my prior patent is applicable only for straight body hull shapes with a block coefficient ˜1. According to the instant invention, the hull, contrary to my aforementioned prior U.S. patent, uses a composite with a light framing on the inside of the composite for the mid-body section which transmits the sea loads to a longitudinal framing or bulkheads, which together with the deck and bottom carry the major loading whereby the light framing on the inside of the composite transmits the sea loads to the longitudinal framing or bulkheads. The instant invention is for Naval combatants that require a curved mid-body section with a block coefficient ˜0.5, such as in a destroyer artistically represented in FIG. 1 of this application. The curved mid-body results in increased fuel efficiency and speed, in addition to other hydrodynamic advantages. The wider mid-body would also result in increased resistance to sea loading and whipping moments. According to this invention, the curved mid-body is made of a hybrid composite and light framing on the inside thereof for transmitting the water pressure loading to an inner straight framing or an inner straight longitudinal bulkhead. The global hull-girder-loads are therefore resisted in this invention by the inner longitudinal-framing or longitudinal bulkheads.